Gilead Sciences Inc., based in Foster City, Calif., has marketed Truvada since 2004 as a treatment for people who are infected with the virus. The medication is a combination of two older HIV drugs, Emtriva and Viread. Doctors usually prescribe the medications as part of a drug cocktail that makes it harder for the virus to reproduce. Patients with low viral levels have reduced symptoms and are far less likely to develop AIDS.

Researchers first reported that Truvada could prevent people from contracting HIV in 2010. A three-year study found that daily doses cut the risk of infection in healthy gay and bisexual men by 44 percent, when accompanied by condoms and counseling. Another study found that Truvada reduced infection by 75 percent in heterosexual couples in which one partner was infected with HIV and the other was not.

Because Truvada is already on the market to manage HIV, some doctors already prescribe it as a preventive measure. But FDA approval would allow the drugmaker Gilead Sciences to formally market its drug for that use.

FDA reviewers on Tuesday said that patients must be diligent about taking the pill every day if using it as a preventative measure. Adherence to the medication was less than perfect in clinical trials, and reviewers said that patients in the real world may forget to take their medication even more than those in clinical studies

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